C Program to Implement Grep, Egrep and Fgrep Commands

Searching for text strings within files is one of the major operations performed in computing. Operating systems like UNIX-like provide high-performance commands like Grep, Egrep, and Fgrep for effective pattern match search across files. This blog post will explain how these commands function. After that comes implementing them in C, which will teach you about string-matching algorithms as well as add to your programming skills.

Understanding Grep, Egrep, and Fgrep:

Grep:

For instance, grep, meaning "Global Regular Expression Print" is a powerful command-line tool that seeks an individual pattern across a file or a flow of text.

The simple form is "grep pattern file", where pattern refers to the regular expression you intend to match, and file is the name of the file you would like to scan.

Egrep:

A more complex variant for this utility is Egreep. It means more or less running Grepe using - E command.

The syntax is the same as Grep: egrep pattern file. Interpreting extended regular expressions is possible with the use of the -E option.

Fgrep:

Fgrep stands for "fixed string GREP" which means it only looks for a particular string in the file. It does not parse ordinary expressions, and it treats the search pattern as a literal string.

The syntax is like Grep: fgrep pattern file. It may be used to search for a particular string without necessarily concerning itself about pattern matching processes.

Implementing Grep, Egrep, and Fgrep in C:

Let us look at the C code to execute these commands now.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <regex.h>
 
void grep(const char *pattern, const char *filename) {
 FILE *file = fopen(filename, "r");
 if (file == NULL) {
 perror("Error opening file");
 exit(EXIT_FAILURE);
 }
 
 char line[MAX_LINE_LENGTH];
 
 while (fgets(line, sizeof(line), file) != NULL) {
 if (strstr(line, pattern) != NULL) {
 printf("%s", line);
 }
 }
 
 fclose(file);
}
 
void egrep(const char *pattern, const char *filename) {
 regex_t regex;
 int reti = regcomp(®ex, pattern, REG_EXTENDED);
 if (reti) {
 fprintf(stderr, "Could not compile regex\n");
 exit(EXIT_FAILURE);
 }
 
 FILE *file = fopen(filename, "r");
 if (file == NULL) {
 perror("Error opening file");
 exit(EXIT_FAILURE);
 }
 
 char line[MAX_LINE_LENGTH];
 
 while (fgets(line, sizeof(line), file) != NULL) {
 if (regexec(®ex, line, 0, NULL, 0) == 0) {
 printf("%s", line);
 }
 }
 
 regfree(®ex);
 fclose(file);
}
 
void fgrep(const char *pattern, const char *filename) {
 FILE *file = fopen(filename, "r");
 if (file == NULL) {

 perror( "Error opening file");
 exit(EXIT_FAILURE);
 }
 
 char line[ MAX_LINE_LENGTH];
 
 while (fgets(line, sizeof(line), file) != NULL) {
 if ( strstr(line, pattern) != NULL) {
 printf("%s", line);
 }
 }
 
 fclose(file);
}
 
int main(int argc, char *argv[] ) {
 if (argc != 4) {
 fprintf(stderr, "Usage: %s [grep|egrep|fgrep] pattern filename\n", argv[0]);
 exit(EXIT_FAILURE);
 }
 
 const char *command = argv[1] ;
 const char *pattern = argv[2];
 const char *filename = argv[3];
 
 if (strcmp( command, "grep") == 0) {
 grep(pattern, filename);
 } else if (strcmp( command, "egrep") == 0) {
 egrep(pattern, filename);
 } else if (strcmp( command, "fgrep") == 0) {
 fgrep( pattern, filename);
 } else {
 Fprintf (stderr, "Invalid command\n");
 exit(EXIT_FAILURE);
 }
 
 return 0;
}

Examples:

Search for the word "error" in a file using Grep:

Search for lines containing the pattern "^[0-9]+" (lines starting with numbers) using Egrep:

Search for the fixed string "apple" in a file using Fgrep:

Conclusion:

In conclusion, the use of Grep, Egrep, and Fgrep commands in C allows for practical learning on text processing and implementing regular expressions. However, string matching is only an illustration to demonstrate that by simple programming, one can solve a lot of practical problems, starting with their most fundamental text search utilities.

The implemented C program demonstrates the versatility of Grep for basic pattern matching, Egrep for extended regular expressions, and Fgrep for literal string searches. By navigating through the code, developers can grasp the intricacies of file handling and string manipulation and even delve into the complexities of regular expression libraries.

Aspiring programmers can use this project as a springboard to explore further enhancements, experiment with additional features, or even integrate these functionalities into larger projects. Knowing how the text search is programmed enhances your coding skills and enables you to use these tools to achieve something useful out there in reality, making the Unix-like systems elegant.

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